A cell line of neural origin (NG108-15) which carries many opiate receptors is used to study the mechanism of action of morphine and morphine-like peptides (endorphins). These materials exert a receptor mediated dual regulation of adenylate cyclase; a rapid inhibition of activity upon which is superimposed a subsequent increase in the amount of total expressible enzyme activity. This, homeostatic, mechanism may account for tolerance to and dependence upon the opiates. Incubation of NG108-15 cell membranes with opioids for 60 minutes leads to a compensatory increase in adenylate cyclase activity. The membrane system reproduces the essetial features of opioid actions in the living cell, and provides a simple, rapid and quantitative measure of physical dependence liability of opioids, as well as opportunities to study the mechanism of dependence in molecular detail. In related studies: rat brain peptidaes have been characterized, one of which may play a role in the biosynthesis of the opioid peptide methionine enkephalin; opioid peptides derived from food proteins, the exorphins, have been further characterized; and it has been found that opioid peptides will penetrate the blood-brain barrier at easily measurable rates.